Chemical compositions of the Ordovician formation water in Tazhong area are dominated by cations as K

+

+ Na

+

and anions like Cl

-

.The water is mostly of CaCl

2

type with salinity ranging from 4.8 to 220.2 g/L and r

Na+

/r

CI-

ratio varing between 0.26 and 0.85

suggesting a small change of water property and a possible origin from mixing of fresh water with sea water after evaporation and inspissation.Influenced by fracturing effect

the plane distribution of formation water salinity is characterized by obvious regionalization ;while affected by regional unconformities

the vertical distribution of ion concentration and salinity is featured in zonation.The relationship between cation concentration of Cl

-

and HCO

3

-

and anion concentration of Na

+

+ K

+

and Ca

2+

indicated that the likely diagenesis process in the study area was mainly dissolutions of NaC1 and carbonate

and that the dolomitization(ferrodolomite)and albitization of calcite resulted in excess of Ca

2+

and deficit of Mg

2+

forming CaCl

2

-type fluid.Formation water in the area can be classified into two groups:normal-pressure high-salinity water and abnormal high-pressure low-salinity water.The former may be related the leaching of atmospheric water and to the water-rock interaction during burial

while the latter may be connected with the closed system formed by the type of water itself.The area between TZ-Ⅰ slope break and TZ-Ⅱ structural zone is considered to have favorable conditions for accumulation and preserving of hydrocarbons.